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a higher energy return than transfer to formed by electron gain through injection- they have been dehalogenated (so that other electron-acceptors such as nitrate, infiltration of non-toxic, electron-rich they chemically resemble Aroclors < nitrite or sulfate. compounds(such as glucose or molasses). 1242), they then can be treated using the A higher yield of energy translates into Adding these electron-rich compounds oxic approach. Several field efforts are a higher rate of microbial growth, and stimulates the indigenous microorgan- evaluating this phased treatment subsequently, a higher rate of contami- isms to extract electrons from the added approach in PCB-contaminated sedi- nant biodegradation. For these reasons, compounds and transfer them to the ments. addition of oxygen to an aquifer is usually electron-deficient contaminants. This Likewise, alternating in situ anoidc/ the preferred means to initiate in situ process usually transforms the contami- oxic treatments may be useful to address bioremediation for the majority of organic nants. aquifers contaminated by highly chlorin- contaminants of concern. Although abundant evidence shows at-ed aliphatic compounds. In some cases Oxygen can be added to a contaminated that a degree of anoxic biodegradation (under controlled conditions), it may be aquifer by sparging the gas directly into occurs naturally in aquifers affected by desirable to enhance the formation of the subsurface. If air is used for sparging, these forms of contaminants, to date, vinyl chloride within such aquifers and the highest concentration achievable there are no documented field efforts then destroy the vinyl chloride (a known within the aquifer is 8 to 10 mg/1. If pure using the anoxic in situ bioremediation human carcinogen) by converting the oxygen is used, the oxygen concentration approach. This promising approach contaminated aquifer to oxic conditions achievable approaches 40 mg/l. This requires field testing. using oxic in situ bioremediation. sparging approach can create elevated Alternating AnoxiclOxic Conditions. A variation of this alternating approach I. dissolved oxygen concentrations within a Several contaminants are treatable using is recommended for treating aquifers con- contaminated aquifer. a combination of anoxic and oxic treat- taminated by more moderately chlorin- However, it is often desirable to inject ments. Highly chlorinated PCBs, such as ated aliphatic compounds such as trichlo- or infiltrate water containing an elevated Aroclors 1254, 1260, are resistant to oxic roethylene(TCE).For this compound,field concentration of dissolved oxygen to influ- biodegradation. research studies have shown that pulsed, ence a larger aquifer region. The injected However, they can become reductively alternating injections of methane and water can be sparged with air or pure dehalogenated under strongly reducing oxygen into the affected aquifer can lead to oxygen,or the water can be amended with conditions created naturally or by using elevated rates of in situ biodegradation of hydrogen peroxide. the anoxic bioremediation approach.Once continued on page 38 Hydrogen peroxide (peroxide) decom- poses in the environment to oxygen and water at a ratio of 2 peroxide molecules to 1 oxygen molecule. Peroxide's advantage a i is that the concentration of the chemical , ■1 can be increased in increments of up to 1,000 mg/1 without appreciably impairing microbial activity. ip An elevated concentration of peroxide can serve two purposes in the aquifer. It - can oxidize reduced aquifer sediments — chemically, and then excess oxygen pro- � - ti?'s . duced by peroxide decomposition can :^ ,•- r ° biodegradation of the ` •�r�. late microbial biode - timu �- S � ��... contaminants.The direct chemical oxida- tion xida tion of reduced sediments may serve to restore quickly oxic conditions within the t contaminated source area, which may be t► _ _ -. valuable in treating severely con- terminated source aquifers. Use of hydro- Qr gen peroxide therefore allows a degree of "< :' treatment flexibility not possible with pure oxygen or air. MULTI-STAGE DIFFUSED AIR STRIPPER Anoxic Conditions. One promising in situ bioremediation method is being .VOC removal to 99.99% • Low Profile—less than a' developed to treat aquifers affected by MTBE to less than 50 ppb overall height electron-deficient compounds such as tetrachloroethylene (PCE) and highly • Skid mounted—standard designs • No trays or packing to plug or foul chlorinated PCBs (polychlorinated bi- phenyls). ENGINEERED SYSTEMS FOR GROUNDWATER AND SOIL VAPOR REMEDIATION In the same way that electron-rich 1-800-242 i`150 compounds can be microbially trans- `'ARBTRO (203) 226-5642 formed by electron loss, electron-deficient CO RPORAT ION 39 Riverside Av Westport,CT 06880 compounds can be microbially trans- Circle 30 on card. 37